The present invention relates to a fast reactor and more particularly to a fast breeder reactor utilizing liquid metallic sodium as a coolant.
In a conventional art of this field are known methods of controlling a reactivity of a fast reactor, for example, one in which a charging condition of neutron absorbers such as control rods is changed and another in which a leak amount of neutrons is controlled by utilizing a reflector. The latter method is particularly effective for a small size reactor core, and in past, an experimental reactor SEFOR was run (SEFOR, Critical experiment: 1969, UO.sub.2 -PuP.sub.2 fuel) Na cooling, Thermal power: 20 MW, Core size: about 566 l). The SEFOR reactor was provided with a reflector area divided into sectors on the outside of the reactor and the divided reflector sectors are moved upward and downward to thereby change relative positional relationship between the reflector sectors and the reactor core portion, thus controlling the leakage of the neutrons.
A military use 10 MWe type power plant in the U.S.A. as a design example of a small size reactor utilizing a reflector has been discussed in an IECE-87 Intersociety Energy Conversion Engineering Conference report by T. A. Moss and E. B. Baumeister, entitled A.Liquid-Metal Reactor/Air Brayton-cycle Option for a Multimegawatt Terrestrial Power (MTP) Plant" Proc. of IECEC-87. p.1596.
The reactor core of this reference is aimed to provide a long-life core by using an enriched uranium oxide fuel. The core power was small 55 MWt (5500 KWe). A reactivity control was done by vertically moving a reflector disposed outside a reactor vessel.
The applicant of the present invention filed a Japanese patent application on Jan. 19, 1989 which discloses a reflector and an elevating mechanism for the reflector disposed on the outer peripheral portion of the reactor vessel (Japanese Patent Laid-open (KOKAI) Specification No. 2-222861).
In the structure disclosed in this laid-open specification, the reflector takes a position opposing a lower end of the reactor core at an initial starting stage of the reactor burning cycle, a reactor core fuel disposed at the lower end of the reactor core is then burnt, and the reactor core fuel is successively burnt from the lower end towards the upper end in accordance with successive elevation of the reflector.
As described, the technology in which the reflector area is provided opposing the reactor core to thereby control the leakage of the neutrons is known as an effective method of controlling the reactivity of the reactor core. However, in a conventional fast reactor, a reflector is disposed at a portion external to the reactor vessel, and for this reason, a large amount of neutrons is irradiated to the reactor vessel, resulting in a problem of narrow selection of the reactor running methods and materials for the reactor vessel. In addition, an elevating device for the reflector is disposed on a portion external to the reactor vessel, such that the arrangement of peripheral equipment around the fast reactor is made complicated and occupies a relatively large space.